The role of macrophages in the immune response of mice to Plasmodium berghei will be evaluated. This will be accomplished by determining whether spleen or peritoneal macrophages are activated during the course of infection, by measuring known markers of activation including enhanced phagocytosis and spreading, levels of plasminogen activor and 5'-nucleotidase, and bactericidal effect against Listeria monocytogenes. The phagocytic ability of normal and activated macrophages, and macrophages of P. berghei-infected animals to ingest parasitized and non-parasitized erythrocytes, which we have previously demonstrated to be sensitized with immunoglobulin; and free parasites, will be studied. In addition, the ability of macrophage activators to alter the course of malarial infection will be analyzed. The information gained in these studies will expand our understanding of the immune response to malaria.